ISRO's twin SPADEX satellites are barely 230metres away from each other, which is the closest they have gotten since they were placed in orbit on December 30, 2024. This indicates significant progress, given that the satellites had drifted almost 6.5kms apart earlier this week, when they were supposed to get at close as 225meters. 

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As of 6pm, Saturday, January 11, ISRO has said that the satellites are in normal health and their sensors are being evaluated. Space docking of two satellites is an intricate, fully autonomous process involving multiple sensors and cameras. 

Two Postponed Docking attempts this week

The first docking attempt planned for January 7 was postponed as ISRO wanted to perform a software test and on-ground simulation.  

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The docking was then pushed to Thursday, January 9. However, on Wednesday night as one of the twin satellites aimed to move closer from 500m to 225m, the distance between the satellites increased drastically. Early on Thursday, private firms Digantara and s2a systems tracked the satellites when they were almost 6.5 kms apart. 

How is ISRO executing SPADEX ?

The primary objective of the SPADEX mission is to develop and demonstrate the technology needed for rendezvous, docking, and undocking of two small spacecraft - SDX01, which is the Chaser, and SDX02, the Target. Each satellite weighs 220kg, and has been hurled into a circular orbit that is 470kms above the earth's surface. 

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At the time of ejection, the final stage of the PSLV rocket gave a small relative velocity between the Target and Chaser spacecraft at the time of separation. Simply put, the target satellite will be travelling slightly faster than the chaser. This incremental velocity will allow the Target spacecraft to build a distance of 10-20 km with the Chaser.

Gradually, the satellites will autonomously reduce the distance between themselves by adjusting their respective velocities. From 20kms, the inter-satellite distance would be lowered to of 5km, 1.5 km, 500m, 225m, 15m, and 3m, ultimately leading to the docking of the two spacecraft. After successful docking and unification, electrical power transfer between the two satellites will be demonstrated. Electrical power transfer is crucial for carrying out in-space robotics, control of the unified spacecraft etc. 

After the primary mission of docking is accomplished, the two satellites will then separate(undock) and start operating as two different satellites, operating their respective payloads for the expected mission life of up to two years.

ISRO Chairman Dr. S. Somanath had told WION that the twin satellites could attempt more docking and undocking, depending on the mission requirement and the availability of fuel on both crafts. 

Why is Space docking so complicated?

When a chaser spacecraft is approaching the target spacecraft, the relative velocity between the two(the difference in velocity between the two) and the distance between the two must be controlled very carefully. If the relative velocity is too high, the docking attempt can fail, causing damage or collisions. If it is too low, it may take too long to complete the approach, which could lead to fuel inefficiencies or mission delays.

When the spacecraft dock, their relative velocity must be near-zero. This means that the two spacecraft must be moving at the same velocity and in the same direction at the time of docking to ensure a smooth mechanical connection without any impact forces. This requires careful coordination and manoeuvring, all done fully autonomously.

Initially the satellites were over 10kms apart from each other. Gradually, the satellites began to autonomously reduce the distance between themselves. The inter-satellite distance had been lowered to 5 km, 1.5 km, 500m and so on. 

WION had earlier reported that SPADEX is ISRO's most complicated mission yet in orbit around the earth. ISRO's twin SPADEX satellites being of small size(and carrying commensurate fuel) also add to the complexity of the mission. It is widely regarded that docking larger satellites with larger quantities of fuel is relatively simpler.

As this is India's maiden docking mission, ISRO is predominantly relying upon the natural drift of the satellites to bring them closer, which is why it is a time-consuming process. It must be noted that SPADEX was launched on 30th December and docking was expected at least a week later. In contrast, large cargo-carrying or human-carrying spacecrafts dock with the International space station in eight hours after being launched into space. This is done by firing the on-board engines of the spacecraft and syncing up in close proximity with the space station, leading to docking.